Vertical interconnect structure, memory device and associated production method

ABSTRACT

The present invention relates to a method for producing a vertical interconnect structure, a memory device and an associated production method, in which case, after the formation of a contact region in a carrier substrate a catalyst is produced on the contact region and a free-standing electrically conductive nanoelement is subsequently formed between the catalyst and the contact region and embedded in a dielectric layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 11/588,769, filed Oct.27, 2006, which claims priority to DE 10 2005 051 973.3 filed on Oct.31, 2005, which are both incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to a vertical interconnect structure, amemory device and an associated production method.

2. Description of Related Art

For the formation of integrated semiconductor circuits, a multiplicityof interconnect structures are required for realizing a wiring or forconnecting the semiconductor components formed in the semiconductorsubstrate. On the one hand, these are horizontal interconnect structuresthat are formed essentially in metallization planes lying above asemiconductor substrate and are isolated from one another by dielectriclayers lying in between. Furthermore, vertical interconnect structuresare required, which either enable a contact-connection from a firstinterconnect or metallization plane to the underlying semiconductorsubstrate and are usually referred to as contacts or, in superordinatemetallization planes or interconnect planes, provide for a connectionbetween said interconnect planes and are usually referred to as contactvias.

Particularly in semiconductor memory devices having volatile ornonvolatile memory elements, a high integration density is necessary inorder to realize a maximum number of items of information per unit area.Furthermore, the production costs are of particular importance forcommercialization.

Usually, in order to realize such vertical interconnect structures orcontacts or contact vias, by means of photolithographic methods, contactholes or openings are formed in the dielectric layers and the contactholes are subsequently filled with electrically conductive fillingmaterial. The very high production costs and also the minimum featuresizes which can be realized only to a limited extent, which prevent moreextensive integration, are disadvantageous in this case in particular onaccount of the photolithographic method. Accordingly, there is a needfor an improved vertical interconnect structure, memory device andassociated methods of production.

SUMMARY

The invention is based on the object of providing a production methodfor vertical interconnect structures, a memory device and an associatedproduction method, an integration density being increased further andthe production costs being reduced further.

In particular, by forming a catalyst on a contact region in a carriersubstrate, subsequently forming a free-standing electrically conductivenanoelement between the catalyst and the contact region, and finallyembedding the free-standing nanoelement in a dielectric layer, it ispossible for the first time to produce vertical interconnect structureswith minimal dimensions in self-aligning fashion and thus in a verysimple manner.

In order to improve an electrical contact-connection, ametal-semiconductor compound, and particularly when using silicon assemiconductor material so-called silicides, may be formed at the surfaceof the contact region.

Furthermore, for the purpose of more extensively decreasing or reducingthe feature size of the vertical interconnect structure, the catalystmay be coagulated by means of e.g. thermal processing. This results in aself-aligning structure miniaturization process that significantlyreduces a cross-sectional area of the vertical interconnect structure.

So-called nanowires, nanotubes or nanofibers are preferably produced asnanoelements or nanostructures. When using silicon nanoelements, acomplete siliciding is preferably carried out, whereby the electricalproperties of the interconnect structure can be improved further and, inparticular, an interconnect resistance is significantly reduced.

As the dielectric layer, SiO₂ is preferably deposited over the wholearea by means of e.g. a CVD method and subsequently planarized as far asthe surface of the nanoelement. This results in an insulation havingoutstanding electrical properties, in which case a layer thickness ofthe dielectric layer can be set particularly simply.

The memory device has at least one memory element and also at least oneselection transistor for selecting the at least one memory element via aword line, a bit line furthermore being connected for reading/writingthe information. In this case, an electrically conductive and initiallyfree-standing nanoelement connects the memory element to the selectiontransistor. Memory devices having particularly high integrationdensities can be formed in this way, with at least one photolithographicmask being saved.

In order to realize so-called elevated source/drain regions,semiconductor layers deposited epitaxially may additionally be formed onthe source/drain regions present in the semiconductor substrate, wherebythe electrical properties of the selection transistors to be realizedcan be improved.

Preferably, as the memory element, a phase change memory element isformed at the surface of the dielectric layer and connected via thenanoelement. On account of the very small cross-sectional areas of thenanoelement, the sufficiently high electric current densities requiredfor programming of a phase change material used in phase change memoryelements can be realized without any problems.

As an alternative, it is also possible to form capacitors and otherresistively switchable memory cells as memory elements at the surface ofthe dielectric layer embedding the nanoelements.

The memory device preferably has two selection transistors for drivingtwo memory elements, a common diffusion region of the selectiontransistors electrically connecting the latter to one another. Aninformation density per unit area can be increased further in this way.

With regard to the method for producing a memory device, preferably atleast one active region is formed in a semiconductor substrate andstrip-type word line stacks with a gate dielectric layer and a gatelayer are subsequently formed on the semiconductor substrate or at thesurface of the active region in order to define at least two contactregions in the active region. The formation of source/drain regions insaid contact regions is followed in turn by formation of at least onefree-standing electrically conductive nanoelement on at least one of thecontact regions, the at least one free-standing nanoelement subsequentlybeing embedded in a dielectric layer. A bit line layer is furthermoreformed at least in the dielectric layer, which is electrically connectedto the further contact region. Finally, at least one memory elementwhich is electrically connected to the at least one nanoelement isformed at the surface of the dielectric layer. A memory device havingextremely high integration density is obtained with minimal costs inthis way.

Preferably, a multiplicity of insular active regions are formed in thesemiconductor substrate and they are divided into three contact regionsby in each case two word line stacks. As a result, two one-transistormemory devices are produced in a cost-effective manner, a contact regionthat lies between the two word line stacks constituting a common drainregion of the selection transistors and respective source regions beingrealized in the remaining contact regions.

Preferably, a common nanoelement is formed on the common drain region, afirst depression subsequently being realized in the dielectric layer inthe region of the common nanoelement, a bit line layer being filled intosaid first depression and the bit line layer subsequently being etchedback in order to form a second depression, in order finally tocompletely fill the second depression with a dielectric fillingmaterial. In this way, it is possible to realize a bit line which iscompletely embedded in the dielectric layer and significantly simplifiessubsequent processing and in particular production of the memoryelements.

As an alternative, however, it is also possible for no nanoelement to beformed on the common drain region, in which case, after the embedding ofthe free-standing nanoelements, etching back of the dielectric layer iscarried out in order to uncover an upper region of the nanoelements,formation of a dielectric etching stop layer at the surface of thedielectric layer and of the uncovered regions of the nanoelements iscarried out, and an auxiliary dielectric layer is formed at the surfaceof the etching stop layer. Afterward, in the region of the common drainregion, a trench is formed in the dielectric layer, the etching stoplayer and the auxiliary dielectric layer as far as the drain region,said trench is filled with the bit line layer, the bit line layer isetched back in order to form a depression and said depression iscompletely filled with dielectric filling material. This again resultsin a bit line which is completely embedded in the dielectric layer andsignificantly simplifies subsequent formation of the memory elements.

Further objects, features and advantages of this invention will becomereadily apparent to persons skilled in the art after a review of thefollowing description, with reference to the drawings and claims thatare appended to and form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 11 illustrate simplified sectional views and also plan viewsof essential method steps in the production of a memory device with aninterconnect structure according to the invention in accordance with afirst exemplary embodiment;

FIGS. 12A and 12B show a simplified sectional view and also anassociated equivalent circuit diagram of a memory device in accordancewith a second exemplary embodiment; and

FIGS. 13A to 16B show simplified sectional views and also plan views forillustrating essential method steps in the production of a memory devicein accordance with a third exemplary embodiment.

DETAILED DESCRIPTION

FIGS. 1A and 1B, respectively, show a simplified sectional view and alsoan associated plan view for illustrating preparatory method steps in theproduction of a vertical interconnect structure and a correspondingmemory device. First, active regions AA are produced in a carriersubstrate 1, which constitutes for example a semiconductor substrate andin particular a monocrystalline silicon semiconductor substrate, by theformation of e.g. a shallow trench isolation 2. The active regions AAare preferably defined in insular fashion, that is to say in the form ofoval or rectangular closed-off regions, by means of an STI method(Shallow Trench Isolation).

Afterward, in accordance with FIG. 1A, a gate dielectric layer 3 maypreferably be formed over the whole area at the surface of the carriersubstrate 1 or of the active regions AA and the shallow trench isolation2. Preferably, a gate oxide having a high quality and a very smallthickness is formed thermally as the gate dielectric layer 3 by means ofa thermal oxidation method. Afterward, an electrically conductive gatelayer 4 is preferably deposited over the whole area, preferably dopedpolycrystalline semiconductor material and in particular polysiliconbeing deposited. Furthermore, a hard mask layer 5 is in turn preferablyformed over the whole area at the surface of the gate layer 4, forexample silicon nitride or silicon dioxide being used.

Afterward, the gate layer 4 and the hard mask layer 5 are patternedusing a resist (not illustrated) by means of photolithographic methodsand the word line stacks illustrated in FIG. 1A are formed. In thiscase, the gate dielectric layer 3 preferably still remains over thewhole area. After the removal of the resist (not illustrated), so-calledspacers 6 are subsequently formed, e.g. an SiN or SiO₂ layer beingdeposited over the whole area and etched back anisotropically until onlythe spacers 6 remain on the sidewalls of the word line stacks.

Using these word line stacks or the additionally present spacers 6 andhard mask layer 5, the corresponding source/drain, connection or LDDimplantations can subsequently be carried out for the purpose ofrealizing the doping regions necessary for a field effect transistor. Onaccount of the insular formation of the active regions AA and the use ofprecisely two strip-type word line stacks which overlie the activeregions AA in such a way that precisely three contact regions aredefined in the active region AA, a memory device having two memory cellsis obtained in which each memory cell respectively has a field effecttransistor and the two memory cells have a common drain contact. A veryhigh integration density is obtained if only for this reason.

FIG. 1B shows a simplified plan view of the illustration in accordancewith FIG. 1A, but the illustration of the gate dielectric layer 3 hasbeen omitted here in order to better illustrate the insular structure ofthe active regions AA in conjunction with the overlying word line stacksor word lines WL1, WL2.

In accordance with FIG. 2, the gate dielectric layer 3 is now removed atleast from the surface of the active regions AA or of the semiconductorsubstrate 1. For this purpose, dilute hydrofluoric acid (HF) ispreferably used for the removal of the SiO₂ gate dielectric layer 3.

In an optional method step in accordance with FIG. 3, a so-calledelevated source/drain region may subsequently be formed in order toimprove the electrical properties of the field effect transistor to beformed, an elevated semiconductor layer 7A being formed at least at asurface of the source/drain regions S and D by means of selectiveepitaxial deposition methods. A selective Si epitaxy is preferablycarried out in this case.

Preferably in accordance with FIG. 4, in addition to the method step inaccordance with FIG. 3 or as an alternative thereto, ametal-semiconductor compound 7B may be formed as the surface of thecontact regions, in particular self-aligning siliciding processes beingemployed when silicon is used as semiconductor material (salicide,self-aligned silicide). Examples of metal-semiconductor compounds 7B ofthis type are CoSi, NiSi, TiSi, etc.

In order to form a selective CoSi_(x) metal-semiconductor compounds 7B,firstly a Co/TiN layer stack may be deposited for example by means ofconventional sputtering methods (PVD, Physical Vapor Deposition). Thisis followed by a first thermal annealing at approximately 500 to 600degrees Celsius with subsequent wet etching of the residual Co and TiNlayers by means of e.g. H₂O₂. Finally, a second thermal annealing of theCoSi metal-semiconductor compound 7B at approximately 600 to 700 degreesCelsius is effected, whereby the metal-semiconductor compound 7Billustrated in FIG. 4 is obtained exclusively at the uncovered surfacesof the semiconductor substrate 1 or the contact regions for the memorydevice.

In this case, blocking masks may be formed in the periphery of asemiconductor component or of an integrated semiconductor circuit inorder to prevent such siliciding.

In accordance with FIGS. 5A to 5C, the vertical interconnect structuresare now produced at the surface of the contact regions or of themetal-semiconductor compounds 7B formed at the source and drain regionsS and D. In order to avoid processing of edge regions, the latter or acorresponding periphery may once again be covered e.g. by means of aresist.

A respective catalyst is now formed selectively at the uncovered contactregions or at the surface of the metal-semiconductor compounds 7B, e.g.an electroless deposition method being carried out. By way of example,Ti, Pd, Pt, Au, Cu, Co, Cr, Hf, Ir, Mn, Mo, Ni, Rh, Ta, W, or Zr isdeposited by means of an electroless deposition method only at thesurfaces of the silicided source and drain regions S and D.

If a resist was used for masking in edge regions or at other locationsof the semiconductor substrate 1, it can now be stripped or removed.Furthermore, the catalyst 8 may subsequently be coagulated for thepurpose of further decreasing a cross-sectional area of the verticalinterconnect structure to be formed. To put it more precisely, at atemperature of e.g. 200-500 degrees Celsius the catalyst is liquefied insuch a way that catalyst droplets form at the surface of themetal-semiconductor compound 7B, which have a smaller area than thesurface areas of said metal-semiconductor compounds 7B. A subsequentstep there involves the formation of a free-standing electricallyconductive nanoelement or a free-standing electrically conductivenanostructure 9 between the catalyst 8 and the contact region orsilicided source and drain region S and D.

Hereinafter the nanoelements or nanostructures 9 formed are, inparticular, nanowires, nanotubes or nanofibers, which are acquiringincreasing importance in semiconductor technology. The literaturereference Lieber, C. M. et al.: “Nanowire Superlattices”, Nano Letters,Volume 2, No. 2, February 2002, discloses for example the production ofsuch nanowires with modulated structures. Furthermore, the literaturereference Cui, Y. et al.: “High Performance Silicon Nanowire FieldEffect Transistors”, Nano Letters, Volume 3, No. 2, 2003, pages 149-152,describes the use of silicon nanowires and carbon nanotubes in theproduction of field effect transistors. Furthermore, the literaturereference Cui, Y., et al.: “Diameter-controlled synthesis ofsingle-crystal silicon nanowires”, Applied Physics Letters, Volume 78,No. 15, 9 Apr. 2001, pages 2214 to 2216, describes the use of siliconnanowires and the production thereof. Finally, the literature referenceMerkulov, V., et al.: “Effects of spatial separation on the growth ofvertically aligned carbon nanofibers produced by plasma-enhancedchemical vapor deposition”, Applied Physics Letters, Volume 80, No. 3,21 Jan. 2002, pages 476-478, discloses the production of vertical carbonnanofibers using an Ni catalyst.

Using one of the methods described above, the free-standing electricallyconductive nanoelements or nanostructures 9 illustrated in FIGS. 5A to5C are now formed in such a way that their height projects significantlyabove the height of the word line stacks with their hard mask layer 5and the spacers 6. The free-standing electrically conductivenanoelements 9 are thus formed in a self-aligning manner and without theuse of an additional lithography between the catalysts 8 and the contactregions or the source and drain regions S and D.

Optionally, said free-standing electrically conductive nanoelements 9,if semiconductor nanoelements and in particular silicon nanoelements areinvolved, may subsequently be silicided, whereby an electricalconductivity is significantly increased and the electrical properties ofthe vertical interconnect structure are thus greatly improved. To put itmore precisely, it is possible, once again using a blocking mask in theperiphery or at parts of the semiconductor substrate which have to beprotected, to apply e.g. a Co/TiN layer stack by sputtering, in whichcase, as in the case of the metal-semiconductor compound 7B, a firstCoSi heat treatment is subsequently carried out, the remaining Co andTiN layers are subsequently subjected to wet etching or removed, and asecond CoSi annealing is finally carried out. Preferably, the entirefree-standing nanoelement 9 is completely silicided in this case. Itgoes without saying that other metals, such as Ni for example, are alsopossible for siliciding, a nickel silicide being formed for the siliconnanoelement 9.

In the—not illustrated—edge regions or connection regions for the wordline stacks, corresponding word line contact regions are, of course,firstly covered in order to reliably prevent an undesirable formation ofnanoelements. Said word line stacks are usually contact-connected in aconventional manner by means of photolithographic patterning processes,corresponding contact holes being etched free and electricallyconductive filling material subsequently being introduced.

In accordance with FIG. 6, the free-standing nanoelements 9 are nowcompletely embedded in a dielectric layer 10, preferably a whole-areadeposition of SiO₂ being carried out with subsequent planarization bymeans of a CMP method which is stopped at the surface or upon reachingthe topmost regions of the nanoelements 9. In this way, thefree-standing nanoelements 9 can be mechanically stabilized andinsulated from one another with a high-quality dielectric material,whereby the electrical and mechanical properties of a semiconductorcomponent are improved. On account of the CMP planarization method used,the catalysts 8, which are no longer required, are simultaneouslyremoved automatically and cost-effectively.

A CVD method (Chemical Vapor Deposition) is preferably used fordepositing the dielectric layer 10, in which case the material may haveTEOS, in particular.

In accordance with FIG. 7, a further resist layer 11 is applied andpatterned photolithographically at the planarized surface of thedielectric layer 10 or of the nanoelements 9. In order to form a bitline, in this case a first depression V1 is etched in the dielectriclayer 10 in the region of the common nanoelement 9 formed above thecommon drain region D, the nanoelement or the nanostructure 9 also beingremoved at the same time. FIG. 8B illustrates a corresponding plan view,FIG. 8C in turn illustrating a sectional view in accordance with thesection B-B.

After this formation of the first depression V1 or the etching of thedielectric layer 10 and the upper region of the nanoelement 9 for thecommon drain region D, in accordance with FIGS. 9A to 9C the resist issubsequently removed and the first depression V1 is filled with a bitline layer 12 and the bit line layer 12 is subsequently etched backagain in order to form a second depression V2. Doped polycrystallinesemiconductor material, and in particular polysilicon, is preferablyused as material for the bit line or the bit line layer 12. Inprinciple, however, it is also possible to use metallic materials, andin particular tungsten, as the bit line layer 12.

Afterward, in accordance with FIG. 10, the second depression V2 iscompletely filled with a dielectric filling material 13, thus resultingin a bit line layer 12 or bit line which is completely embedded in thedielectric layer 10 and is connected to the common drain region D via acommon nanoelement 9. Preferably, as the dielectric filling material 13,SiO₂ is once again deposited over the whole area by means of a CVDmethod and a renewed planarization method is carried out, the upperregions of the further nanoelements 9 serving as stop layers.

In accordance with FIG. 11, the memory elements are now formed at theplanarized surface of the dielectric layer or of the uncoverednanoelements 9, so-called phase change memory elements PW1 and PW2 beingformed in accordance with the first exemplary embodiment.

Such phase change memory elements use materials which, with regard totheir electrical properties, are capable of changing over reversiblyfrom one phase to another phase. By way of example, such materialschange between a phase ordered in amorphous fashion and a phase orderedin crystalline or polycrystalline fashion. In particular a resistance ora conductance of such a material differs greatly in these two differencephase states.

Therefore, phase change memory elements usually use such phase changematerials, which for example constitute alloys of elements in group VIof the periodic table and are referred to as so-called chalcogenides orchalcogenide materials. Accordingly, such phase change materials areunderstood hereinafter to be materials which can be changed over betweentwo different phase states having different electrical properties(resistances).

Currently the most widespread chalcogenides or phase change materialscomprise an alloy of Ge, Sb and Te (GST, Ge_(x)Sb_(y)Te_(z)). Ge₂Sb₂Te₅is already used in a large number of phase change memory elements and isfurthermore known as a material for rewritable optical storage media(e.g. CDs, DVDs, etc.).

The changes in the resistance of phase change materials are utilized inorder, for example, to create nonvolatile memory devices (NVM, NonVolatile Memory) and to store information. Accordingly, such materialshave a higher resistance in the amorphous phase than in the crystallineor polycrystalline phase. Accordingly, a phase change material may beused as a programmable resistor, the resistance magnitude of which canbe reversibly altered depending on its phase state.

An overview of such phase change materials is disclosed for example inthe literature reference S. Hatkins et al.: “Overview of phase-changechalcogenide non-volatile memory technology”, MRS Bulletin/November2004, pages 829-832.

A change in the phase in such materials may be caused by a localincrease in a temperature. Both phase states are usually stable below150 degrees Celsius. Above 300 degrees Celsius, rapid crystal nucleationtakes place, resulting in a change in the phase state to a crystallineor polycrystalline state, provided that such a temperature is presentfor a sufficient length of time. To return the phase state to theamorphous state, the temperature is increased to above the melting pointof 600 degrees Celsius, followed by very rapid cooling. Both criticaltemperatures, i.e. both for the crystallization and for the melting, canbe generated using an electric current which flows through anelectrically conductive connection electrode with a predeterminedresistance and is in contact with or in the vicinity of the phase changematerial. The heating is in this case carried out by so-called Jouleheating.

In accordance with FIG. 11, such a phase change material 14 is nowdeposited over the whole area at the planarized surface or thenanoelements 9, a PVD or CVD method preferably being carried out.Afterward, a phase change electrode layer 15 is once again preferablydeposited over the whole area at the surface of the phase changematerial layer 14, TiN being deposited, for example. Finally, this layerstack is patterned using conventional photolithographic methods, wherebythe phase change memory elements PW1 and PW2 illustrated in FIG. 11 canbe realized at the surface regions of the nanoelements.

Whereas the very high programming currents required, which are necessaryfor the change in the phase state, constitute a significant disadvantagein conventional phase change memory elements, for the first time andwithout additional costs it is possible, on account of the nanoelements9 used, to realize very small contact areas to the phase change memoryelement, which enable the current path to be spatially delimited to agreat extent and thus enable the phase change memory elements PW1 andPW2 to be programmed even with very small programming currents. Thisresults in a nonvolatile memory device with an extremely highintegration density which can be produced extremely cost-effectively andcan be operated with very low programming currents.

Further resistively switchable memory elements may also be used besidesthe phase change memory element described above. They include MRAMmemory elements (Magnetic RAM), for example, in which a magnetic layercan be programmed for storing items of information and be read.Furthermore, CBRAM memory elements (Conductive Bridging RAM) are known,for example, in which a conductivity can be set in a solid electrolyteor chalcogenide by producing conductive microbridges. The literaturereference G. Müller et al.: “Status and Outlook of Emerging NonvolatileMemory Technologies”, IEEE 2004, describes corresponding memoryelements.

FIGS. 12A and 12B show a simplified sectional view of a memory device inaccordance with a second exemplary embodiment, stacked capacitors C1 andC2 being used as volatile memory elements instead of the phase changememory elements. In this case, identical reference symbols designatelayers and elements identical or corresponding to those in FIGS. 1 to11, for which reason a repeated description is dispensed with below.

In accordance with the equivalent circuit diagram according to FIG. 12B,the memory device according to the invention thus has two selectiontransistors T1 and T2, which are respectively connected to a word lineWL1 and WL2 and the common drain of which is connected to a common bitline BL. As memory elements, the capacitors C1 and C2 are connected tothe respective source regions of the selection transistors T1 and T2,which preferably comprise field effect transistors.

The same method steps as illustrated in FIGS. 1 to 10 are once againcarried out in accordance with the second exemplary embodiment, forwhich reason the detailed description of said method steps is dispensedwith below. According to a method step as is illustrated in FIG. 10, inaccordance with the second exemplary embodiment firstly an etching stoplayer 16 is formed over the whole area, a thin SiN etching stop layerbeing deposited by means of a CVD method, by way of example. A furtherdielectric layer 17 is subsequently formed at the surface of saidetching stop layer 16, an SiO₂ layer preferably again being deposited bymeans of a CVD method.

Using a resist (not illustrated), a trench or depressions is or aresubsequently formed into said further dielectric layer 17 as far as thesurface of the dielectric layer 10 at least in the region of the furthernanoelements 9 for the source regions S, the further nanoelements 9being uncovered at their upper surface.

A first capacitor electrode layer 18 is subsequently formed at least atthe bottom of the trench surface or the depressions, preferably apolycrystalline semiconductor layer and in particular doped polysiliconbeing deposited by means of a CVD or ALD (Atomic Layer Deposition)method and being patterned by means of a CMP method. As an alternative,a metal layer such as e.g. TiN may also be used as the first electrode.A capacitor dielectric layer 19 is subsequently formed at least at thesurface of the first capacitor electrode layer 18, a so-called high-kdielectric (e.g. Al₂O₃) preferably being deposited over the whole areaby means of ALD methods. Finally, a second capacitor electrode layer 20or the capacitor counterelectrode is deposited over the whole area, aTiN layer preferably being deposited by means of a CVD or ALD method.

The memory device in accordance with a second exemplary embodiment asillustrated in FIG. 12A is obtained in this way, which once again has ahigh integration density and can be produced in a particularlycost-effective manner. The significant cost advantages arise inparticular on account of the vertical interconnect structures which aregrown in a self-aligning manner at the surface of the source and drainregions S and D.

Furthermore, it is also possible to use capacitors as nonvolatile memoryelements, the capacitor dielectric comprising e.g. a ferroelectricmaterial, e.g. PZT (lead zirconium titanate). Memory elements of thistype are usually referred to as FeRAM memory elements (ferroelectricRAM). The literature reference G. Müller et al.: “Status and Outlook ofEmerging Nonvolatile Memory Technologies”, IEEE 2004, describescorresponding memory elements.

FIGS. 13A to 16B show further sectional views and also plan views forillustrating essential method steps in the production of a memory devicein accordance with a third exemplary embodiment, identical referencesymbols designating elements identical or corresponding to those inFIGS. 1 to 12, for which reason a repeated description is dispensed withbelow.

In accordance with FIGS. 13A and 13B, firstly insular active regions AAare once again formed at the surface of the carrier substrate 1 by meansof an STI method and strip-type word line stacks or word lines WL1 andWL2 are arranged there above. In contrast to the first and secondexemplary embodiments, however, the active region AA has a laterallyprotruding projection in its central region, in which a common drainregion D will be formed later, said projection being provided for directcontact-connection of the common drain region D by a bit line layer thatis to be formed later.

In accordance with FIG. 14, in this case, after forming the spacers 6and carrying out the various implantations for forming the source anddrain regions S and B, a drain mask M is preferably formed in strip-typefashion between the word line stacks WL1 and WL2 for the purpose ofcovering at least the common drain region D.

Preferably, a so-called SiLK is in this case deposited over the wholearea and patterned in accordance with FIG. 14, reference being made inparticular to the literature reference A. Birner et al.: “A fourthmaterial: Thermally stable organic gap-fill spin-on-polymer enabling newintegration concepts”, IEDM Tech. Dig., December 2003, pages 665 to 668.This mask layer M, which is also used as a blocking mask for latersiliciding, may likewise be formed in the edge regions or the peripheryof a respective semiconductor circuit.

In accordance with FIG. 15, after the removal of the gate dielectriclayer 3 at the uncovered surfaces of the carrier substrate 1 using, forexample, dilute hydrofluoric acid (HF), a metal-semiconductor compound7B is once again implemented at the uncovered contact regions or at theuncovered source regions S, in which case it is possible to form e.g.CoSi_(x) as metal-semiconductor compound 7B. Reference is made here tothe comparable process steps in accordance with the first and secondexemplary embodiments.

Preferably, however, prior to the second CoSi annealing step, the SiLKmask is removed selectively with respect to the SiO₂ and Si₃N₄ and thesecond annealing step is then carried out. Afterward, the same steps asin FIGS. 5 and 6 are once again carried out, but after the planarizationstep as far as the upper surfaces of the nanoelements 9 in accordancewith FIG. 6, now the dielectric layer 10 is etched back in order touncover an upper region of the nanoelements 9 and a dielectric etchingstop layer 21 is subsequently formed at the surface of the dielectriclayer 10 and the uncovered regions of the nanoelements 9.

Preferably Si₃N₄ is once again deposited over the whole area as thedielectric etching stop layer 21. An auxiliary dielectric layer 22 issubsequently formed over the whole area at the surface of the etchingstop layer 21, an SiO₂ deposition with subsequent planarizationpreferably being carried out.

In order to form the bit line, in the region of the common drain regionD, a trench is subsequently formed in the dielectric layer 10, theetching stop layer 21 and the auxiliary dielectric layer 22 down to thedrain region D and the trench is firstly completely filled with the bitline layer 12. Preferably polycrystalline semiconductor materials, andin particular doped polysilicon, are once again used as materials forthe bit line layer 12. In principle, however, it is also possible to usemetallic materials, and in particular tungsten.

The bit line layer 12 is once again etched back in its upper region inorder to form a further depression and the further depression is finallyfilled completely with the dielectric filling material 13. In this case,materials comparable to those in the first or second exemplaryembodiment are once again used and a whole-area deposition of SiO₂ withsubsequent planarization is preferably carried out.

Finally, as in the first exemplary embodiment, phase change memoryelements PW1 and PW2 may once again be formed at the locations of theuncovered nanoelements 9 or at the surface of the auxiliary dielectriclayer 22, the memory device being completed in accordance with the thirdexemplary embodiment.

Once again, a memory device with a maximum integration density in whichcontact-connection is formed essentially in a self-aligning manner isobtained, in accordance with FIG. 16A.

FIG. 16B shows a simplified plan view with a sectional illustration A-Aof the sectional view illustrated in FIG. 16A in order to illustratethis third exemplary embodiment.

In accordance with this third exemplary embodiment, it is consequentlypossible to employ a so-called dual damascene bit line process, whichenables the costs to be reduced further. It goes without saying thatfurther additional hard masks can also be used in accordance with thethird exemplary embodiment.

The present invention has been described on the basis of phase changememory elements and capacitors as memory elements. However, it is notrestricted thereto and also encompasses alternative memory elements inthe same way. Furthermore, the invention has been described on the basisof two one-transistor memory cells in which two selection transistorsare formed in an active region. However, it is not restricted theretoand also encompasses other types of memory cells in the same way.

As a person skilled in the art will readily appreciate, the abovedescription is meant as an illustration of implementation of theprinciples this invention. This description is not intended to limit thescope or application of this invention in that the invention issusceptible to modification, variation and change, without departingfrom the spirit of this invention, as defined in the following claims.

We claim:
 1. A method for producing a memory device comprising the stepsof: forming a multiplicity of insular active regions in a semiconductorsubstrate, forming two strip-type word line stacks with a gatedielectric layer and a gate layer on the semiconductor substrate in sucha way that three contact regions are formed in the insular activeregions, forming a common drain region in a contact region lying betweenthe two word line stacks and forming source regions in the remainingcontact regions, forming a common nanoelement on the common drainregion, and forming additional nanoelements on the source regions,embedding the common nanoelement in a dielectric layer, forming a firstdepression in the dielectric layer in the region of the commonnanoelement, filling a bit line layer into the first depression, the bitline layer being electrically connected to the contact region lyingbetween the two word stacks, etching the bit line layer back to form asecond depression, filling completely the second depression with adielectric filling material, and forming at least one memory element atthe surface of the dielectric layer in such a way that it iselectrically connected to the additional nanoelements.
 2. The method asclaimed in claim 1, further comprising the step of selectivelydepositing an elevated semiconductor layer epitaxially as the surface ofthe contact regions.
 3. The method as claimed in claim 1, furthercomprising the step of forming a metal-semiconductor compound as thesurface of the contact regions.
 4. The method as claimed in claim 1,further comprising the steps of forming a catalyst on at least onecontact region and depositing the nanoelement in free-standing fashionbetween catalyst and contact region.
 5. The method as claimed in claim4, wherein the catalyst is coagulated in order to reduce its area. 6.The method as claimed in claim 1, further comprising the step of formingfree-standing nanowires, nanotubes or nanofibers when the at least onefree-standing electrically conductive nanoelement is formed.
 7. Themethod as claimed in claim 1, further comprising the steps of depositingSiO₂ over the whole area as the dielectric layer and planarizing theSiO₂ as far as the nanoelement.
 8. The method as claimed in claim 1,wherein the free-standing nanoelements are higher than the word linestacks.
 9. The method as claimed in claim 1, further comprising the stepof forming a second dielectric layer and at least one stacked capacitorcomprising a memory element, the stacked capacitor, having a firstcapacitor electrode layer, a capacitor dielectric layer and a secondcapacitor electrode layer.
 10. The method as claimed in claim 1, furthercomprising the steps of forming a phase change material layer at thesurface of the dielectric layer and the nanoelements and forming a phasechange electrode layer as a phase change memory element.
 11. The methodas claimed in claim 1, wherein the at least one free-standingelectrically conducting nanoelement comprises silicon.
 12. The method asclaimed in claim 11, further comprising the step of fully siliciding thesilicon nanoelements when the at least one free-standing electricallyconductive nanoelement is formed.
 13. A method for producing a memorydevice comprising the steps of: forming a multiplicity of insular activeregions in a semiconductor substrate, forming two strip-type word linestacks with a gate dielectric layer and a gate layer on thesemiconductor substrate in such a way that three contact regions areformed in the insular active regions, forming a common drain region in acontact region lying between the two word line stacks and forming sourceregions in the remaining contact regions, forming nanoelements on thesource regions, forming a dielectric layer over the common drain regionand the nanoelements, etching back the dielectric layer to uncover anupper region of the nanoelements; forming a dielectric etching stoplayer at the surface of the dielectric layer and of the uncoveredregions of the nanoelements; forming an auxiliary dielectric layer atthe surface of the etching stop layer; forming a trench in the region ofthe common drain region in the dielectric layer, the etching stop layerand the auxiliary dielectric layer as far as the drain region, filling abit line layer into the trench, etching the bit line layer back to forma depression, and filling completely the depression with dielectricfilling material.